Recently, so-called flat panel sensors have become popular as X-ray digital imaging apparatuses. These sensors have the potential to replace conventional imaging systems using films, and are designed to output data obtained by directly digitizing an X-ray image, and hence are expected to be developed in a wide range of applications. Many such flat panel sensors use amorphous silicon, and are required to increase imaging areas and resolution. At the same time, such sensors are required to achieve high sensitivity and high S/N ratio.
On the other hand, imaging units have recently been required to be reduced in size, and cassette type imaging units and the like have also been proposed (Japanese Patent Laid-Open No. 2003-248060). An imaging unit incorporates the above flat panel sensor, its driving circuit, a signal detection circuit, a digital circuit, a power supply circuit, and the like. Of these components, the power supply circuit is, in particular, a bottleneck in achieving a compact imaging unit. The power supply circuit generally acquires power from a commercial AC power supply, and hence includes a transformer for converting AC to DC and the like. As a consequence, the overall circuit increases in size. Mounting such a power supply circuit in an imaging unit makes it impossible to achieve a reduction in size. For this reason, there has been proposed a method of separating a power supply circuit portion which converts AC to DC from an imaging unit, causing this portion as a discrete power supply unit to generate a predetermined DC voltage, and applying it to the imaging unit through a power cable of several meters.
The imaging unit needs to apply different DC voltages to several circuits like those described above. Generating these voltages within the above discrete power supply unit and applying them to the imaging unit pose many problems in practical use, including drop voltage in the cable, superimposition of noise, and the like, if the length of a power cable is several meters or more. For this reason, the following method is employed: power is supplied at a relatively high DC voltage as a uniform voltage from the power supply unit, and a switching power supply (to be referred to as an SW power supply hereinafter) such as a DC/DC power supply is provided in the imaging unit to generate various voltages and apply them to the respective circuits. Although DC/DC power supplies have been increasingly miniaturized owing to recent technical advances, they produce conductive noise and radiation noise because they are SW power supplies. Such noise may be superimposed on peripheral circuits, and more particularly a sensor panel, amplifier IC, and A/D conversion circuit to affect images. Leakage magnetic field noise which is radiation noise may be magnetically coupled to peripheral circuits, and more particularly a detection system including a flat panel sensor and amplifier IC, to generate an induction noise voltage, thereby seriously affecting image quality.
In addition, in order to miniaturize the imaging unit, it is necessary to place the above flat panel sensor and other peripheral circuits closer to the DC/DC power supply as well as to reduce the size of the DC/DC power itself. Recently, with miniaturization of imaging units, the spatial distances between a power supply and a sensor and its peripheral circuits, which are incorporated in an imaging unit, have decreased more and more. As a consequence, the sensor has become susceptible to the influence of magnetic coupling, and more particularly leakage magnetic field noise from the DC/DC power supply. For this reason, noise is superimposed on a read signal from the sensor to cause line noise on images. It is therefore indispensable to provide countermeasures against noise in the DC/DC power supply.
In general, as measures to suppress electromagnetic wave noise such as leakage magnetic field noise from an SW power supply, the following measures have been executed: measures associated cable laying, measures at a component level such as a transformer, preventing magnetic field leakage by shielding the entire power supply, and the like. With only measures associated with cable laying, the noise suppressing effect is small. In addition, it is difficult to confine a leakage magnetic field with a magnetic field shield. In addition, this makes it difficult to achieve reductions in size and weight. As a measure at a component level, for example, rounding a switching waveform can reduce noise. This, however, reduces the conversion efficiency. In addition, an efficiency loss will lead to heat generation. In consideration of the problems associated with size, shape, weight, cost, heat, and the like, it is difficult to prevent the influences of leakage magnetic fields while miniaturizing the overall apparatus.
In addition, the above problems also originate from the fact that a very high level of measures against noise is required for a power supply because the signal level to be handled corresponds to microvolt-level voltages. For example, of general DC/DC power supplies, even high-quality power supplies cause ripples of several tens of millivolts and spike noise (conductive noise). Even noise of such a level is superimposed on an image signal through a route for supplying power to a detection circuit and affects an image.
With regard to ripple noise leaking at a main oscillation frequency, there are available measures to be taken at the noise source, a measure of adding an output filter circuit, and the like. A certain degree of effect can be obtained by such measures. In addition, in a low frequency band including a main oscillation frequency and the like, the low leakage ratio of noise from a signal amplifier power supply to signals effectively serves to reduce noise. However, high-frequency noise is produced at a switching point (ON/OFF switching point) in an oscillation signal for the DC/DC power supply. It is difficult to obtain effective results on such high-frequency noise by using the above measures.
Against high-frequency noise, a countermeasure component such as a ferrite core may be mounted in the apparatus. However, in consideration of reductions in size and weight and cost, it is not preferable to mount such an additional countermeasure component. In addition, measures to reduce noise may sometimes lead to a reduction in conversion efficiency, making it difficult to execute effective measures.